The demand for high speed data connections is growing every day. The cost and delay associated with installing electrical and optical cables to carry high speed data are often greater than the market can bear. As an alternative, wireless broadband access (WBA) services have been developed which allow for transmission of high speed data over the wireless channel. WBA service is typically offered in a relatively high frequency band (such as 18 to 40 GHz) so that the operational bandwidth can be very broad such as about (50 MHz) allowing data rates of 200 megabits per second (MBPS) and higher. Many countries have specifically allocated spectrum for the WBA services. However, the allocated spectrum is not consistent and varies from country to country.
One key element of any WBA system is the radio card which transmits and receives the high frequency signals over the wireless link. Designing a new radio card for each possible frequency band is expensive and time consuming.
As the WBA service market grows, more pre-packaged parts are available to radio designers. For example, Hittite Microwave Corporation™ makes a line of packaged GaAs parts for use in the 20 to 40 GHz range. One part made by Hittite™ is the HMC264LM3 GaAs MMIC Sub-Harmonic SMT Mixer 20–30 GHZ™. The part comes in a leadless chip carrier package. The specification sheet for the part recommends mounting the device on Rogers RO4003™ material using 0.5 oz. copper.
Rogers RO4003™ material is made by Rogers Corporation™. The material is a glass reinforced hydrocarbon thermoset laminate. FIG. 1 illustrates a Rogers™ multi-layered printed circuit board (PCB). The stack up 100 of the Rogers™ PCB includes a core material I with a plurality of conductive and non-conductive layers at its upper and lower surfaces. At the upper surface of the core 1, the stack up 100 includes a first inner conductive layer 2A, a first inner non-conductive layer 3A, and a first Rogers™ core 5A with a first outer conductive layer 4A and a first outer-most conductive layer 6A. At the lower surface of the core 1, the stack up 100 includes a second inner conductive layer 2B, a second inner non-conductive layer 3B, and a second Rogers™ core 5B with a second outer conductive layer 4B and a second outer-most conductive layer 6B. As shown in FIG. 1, the stack up 100 further includes vias 7A, 7B, 8A, 8B, and an opening 9.
Several difficulties arise with the Rogers™ multi-layer PCB. A typical radio includes both a very high radio frequency (RF) portion and a lower intermediate frequency (IF) portion. However, a composite PCB made from a RO4000 series or other ceramic or Teflon laminate does not provide a good substrate for the lower frequency operation. Lower frequency designs work better with thicker dielectric materials, i.e., larger metal features provides better tolerance at IF, while the high frequency materials are typically very thin. An assembly incorporating a daughter board to carry the RF or IF signaling can be used to boost signal performance, but this solution is more expensive and difficult to manufacture than a single board design.
In addition, the Rogers™ multi-layered PCB requires a custom manufacturing process. FIG. 2 is a flowchart illustrating the manufacturing process for the Rogers™ multi-layer PCB. First, the core 1 is provided with the first inner conductive layer 2A at a first surface and the second inner conductive layer 2B at a second surface opposite to the first surface, via step 201. Next, the first inner non-conductive layer 3A is applied to the first inner conductive layer 2A, via step 202, and a second inner non-conductive layer 3B is applied to the second inner conductive layer 2B, via step 203. Then, a first Rogers™ core 5A, with a first outer conductive layer 4A on one side and a first outer-most conductive layer 6A on the other side, is applied to the first inner non-conductive layer 3A, via step 204. A second Rogers™ core 5B, with a second outer conductive layer 4B on one side and a second outer-most conductive layer 6B on the other side, is applied to the second inner non-conductive layer 3B, via step 205. All of the above layers are then simultaneously laminated, via step 206. However, this manufacturing process is expensive since specialized, custom processing steps are required.
Conventional radio cards that integrate RF and IF have several other disadvantages. For example, the high frequency materials used in the radio cards are expensive. A PCB which uses these materials is much more expensive to design, prototype, iterate and produce. Even if these high frequency materials are used with standard PCB materials, compatibility and responsibility issues limit their viability. The high frequency materials are typically manufactured by a company different from the one that manufactures the standard PCB materials. If the high frequency material delaminates from the lower frequency materials produced by the other company, no single company is responsible for the failure. This increases both the financial and technical risks associated with the use of a composite PCB.
Also, ceramic laminates are rigid. During the PCB fabrication process, the rigid high frequency material is laminated to a standard rigid FR4 core material using both heat and pressure. A special glue or prepreg material is placed between the high frequency material and the FR4 core. The assembly is pressed between two heated plates. The glue melts and deforms to provide a mechanical connection to the assembly. Because both the high frequency material and the stand FR4 core are rigid, stress builds up between the plates and both surfaces of the PCB at various locations during the lamination process. As the PCB is cooled and removed from the press, the PCB seeks to relieve these stresses by deforming. Because the location of the stresses varies based on the design features impressed on the PCB, the PCB may become concave, convex, wavey or twisted. Predicting the effects of the stress is extremely difficult. Relieving the stress can require redesign of the RF and IF layout of the PCB, or require adjustment in the machinery. Thus, use of rigid ceramic laminates may result in a PCB which is not flat and which causes a variety of negative effects to the fully assembled board.
Another problem with conventional radio cards concerns the mounting of a microstrip filter to the PCB. At high frequency bands, it is convenient to use a microstrip to create certain circuit elements such as transmission lines, couplers and filters. As the frequency band at which the system operates varies, the filtering requirements imposed on the radio card also vary. Thus, in a reusable, versatile PCB design, the microstrip filters cannot be printed directly on the PCB, and the tolerance on a low cost PCB is not good. Instead, the filters can be designed on a surface mount substrate or leadless surface mount substrate. One common substrate is alumina. A surface mount printed filter is soldered to the PCB to provide both signaling and a ground plane to the filter. However, when the conventional PCB is not flat, the surface mount printed filter may not properly attach or it may detach from the PCB when the populated PCB is installed in its housing.
Accordingly, there exists a need for an improved multi-layered integrated RF/IF circuit board. The improved board should provide good RF and IF performance on a single board. Its manufacturing process should be inexpensive, requiring little or no custom processing. The present invention addresses such a need.